Iceland closed airspace over the Bardarbunga volcano as a precaution Saturday after the volcano began erupting under the ice of Europe's largest glacier.

Authorities did not shut down airspace over the rest of the nation as Iceland raised its aviation alert to its highest level of red and put the Coast Guard on alert after detecting a small subglacial eruption of lava from the volcano, which is beneath the Vatnajokull glacier.

Iceland's red aviation alert, the highest on a five-point scale, is a warning that an eruption could be imminent. However, scientists who flew over the volcano spotted no visible signs of an eruption at the surface that would spew ash and steam into the atmosphere.

The volcano has been rocked by thousands of earthquakes in the past week.

"Both the thickness of the ice at the possible contact point ... and the volume of lava in possible contact with ice are highly uncertain," Iceland's Meteorological Office in Reykjavik said in a statement on its website.

"It is also possible that the lava will not break through the ice, and the eruption could remain subglacial," the agency said.

Vulcanologist Melissa Pfeffer told the Associated Press that seismic data shows lava from the volcano is melting ice beneath the Vatnajokull glacier. It's not still clear whether the eruption will melt the ice and send steam and ash into the air.

Ash from Iceland's Grimsvotn volcano canceled flights in Scotland, northern England and Germany in May 2011. The eruption of the Eyjafjallajokull volcano in April 2010 forced the cancellation of more than 100,000 flights.

Airlines are monitoring the current situation. Since 2010, aviation regulators have changed policies about flying through ash. They provide detailed information about where ash clouds are and how dense they are and then leave the final decision up to airlines and regulators.

The chunk of closed airspace over the volcano extends over the North Atlantic, and Virgin Atlantic said a London-do-San Francisco flight took a detour to avoid it.

Nicholas Wyke, a spokesman for continental air traffic controller Eurocontrol, told The Associated Press in an e-mail that it was difficult to estimate how many flights typically crossed that airspace, noting that trans-Atlantic flights vary their routes based on wind conditions.

"It is unlikely that this danger area will have any significant effect on north Atlantic traffic," he said.

The Bardarbunga volcano is 15.5 miles wide and rises about 6,234 feet above sea level and is beneath Europe's largest glacier. It last erupted in 1996.

Authorities evacuated several hundred people earlier this week from the highlands north of the Vatnajokull glacier as a precaution.